Law of definite proportions (Proust) — Hinglish notes

1.1.9 · Chemistry › Matter, Measurement & the Mole

YEH HAI KYA?

Key word hai compound. Yeh mixtures ke baare mein nahi hai — ek mixture (jaise namak paani mein milaya) kisi bhi proportion mein ho sakta hai. Ek compound mein chemically bonded atoms hote hain whole-number counts mein.

YEH SACH KYU HAI? (First-principles reasoning)

Maano ek compound mein $n_{A}$ atoms hain element $A$ ke (atomic mass $m_{A}$ ) aur $n_{B}$ atoms hain $B$ ke (atomic mass $m_{B}$ ) har molecule mein.

$\frac{mass of A}{mass of B} = \frac{n _{A} m _{A}}{n _{B} m _{B}}$

Right side ki har term ek constant hai — $n_{A}, n_{B}$ fixed chhote integers hain, $m_{A}, m_{B}$ fixed atomic masses hain. Toh ratio ek fixed number hai. Yahi law hai.

ISKO USE KAISE KAREIN (workflow)

Ek sample se elements ka mass ratio nikalo.
Kyunki law kehta hai yeh ratio constant hai, kisi bhi doosre sample mein masses predict karo.
Agar numbers kisi fixed ratio se match nahi karti → aap koi pure compound nahi dekh rahe (ya data ek mixture / experimental error hai).

Figure — Law of definite proportions (Proust)

Worked Examples

Common Mistakes (Steel-manned)

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho har "water Lego" exactly 2 laal bricks (hydrogen) aur 1 neeli brick (oxygen) ko snap karke banaya jaata hai. Aap kabhi 2.5 laal bricks se water Lego nahi bana sakte — bricks split nahi hoti! Toh chahe kitne bhi water Legos banao, laal bricks ka neeli bricks se dhair hamesha same proportion mein weighs karega. Isliye kahin ka bhi paani hamesha $8$ grams oxygen hota hai har $1$ gram hydrogen ke liye.

Active Recall

Law of Definite Proportions kya kehta hai?

Ek pure compound hamesha same elements ko same fixed ratio by mass mein contain karta hai, chahe source ya method of preparation kuch bhi ho.

Yeh kisne aur kab propose kiya?

Joseph Proust, 1799.

Ek compound ka mass ratio constant kyun hota hai?

Har molecule mein har element ke atoms ki ek fixed whole number hoti hai, aur har atom ki ek fixed mass hoti hai, isliye

m_{A} / m_{B} = n_{A} m_{A}^{a t o m} / n_{B} m_{B}^{a t o m}

ek constant hai.

Kya yeh law mixtures par apply hota hai?

Nahi — sirf pure compounds par; mixtures kisi bhi proportion mein ho sakte hain.

Paani 8:1 O:H by mass hai. 2.5 g H ke saath kitna O combine hoga?

8 \times 2.5 = 20

g oxygen.

CO aur CO₂ mein same elements hain lekin alag ratios hain — kya yeh law violate karta hai?

Nahi. Har compound ka individually ek fixed ratio hota hai. Alag-alag compounds ka differ karna Law of Multiple Proportions hai.

Agar aap hydrogen mein excess oxygen add karo, toh kya paani ki composition badlegi?

Nahi; excess O bacha hua reactant hoga. Bana hua paani 8:1 rehta hai.

Do Cu-oxide samples Cu:O = 4.00 aur 4.00 dete hain. Same compound?

Haan — identical fixed mass ratios same compound indicate karte hain.

Connections

Dalton's Atomic Theory — atomic basis (fixed atom counts) jo is law ko explain karta hai.
Law of Conservation of Mass — Lavoisier ka law; mass conserved aur ratio mein fixed hoti hai.
Law of Multiple Proportions — same elements ke alag-alag compounds ke liye sister law.
The Mole Concept — mass ratios mole ratios aur formulas tak le jaate hain.
Empirical and Molecular Formula — fixed ratios se hum chemical formulas deduce kar sakte hain.
Percentage Composition — direct consequence: har element ka fixed % by mass.